JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A11201, doi:10.1029/2005JA011094, 2005
Signature of Saturn’s auroral cusp: Simultaneous Hubble Space
Telescope FUV observations and upstream solar wind monitoring
Jean-Claude Gérard,1 Emma J. Bunce,2 Denis Grodent,1 Stanley W. H. Cowley,2
John T. Clarke,3 and Sarah V. Badman2
Received 18 February 2005; revised 7 July 2005; accepted 29 July 2005; published 11 November 2005.
[1] Model simulations by Bunce et al. (2005a) have shown that direct precipitation of
electrons in Saturn’s dayside cusp regions is not capable of producing significant FUV
aurora. Instead, they suggested the possibility that the FUV bright emissions sometimes
observed near noon are associated with reconnection occurring at the dayside
magnetopause, possibly pulsed, analogous to flux transfer events seen at the Earth. Pulsed
reconnection at the low-latitude dayside magnetopause when the IMF is directed
northward (antiparallel to Saturn’s magnetic field lines) is expected to give rise to pulsed
twin-vortical flows in the magnetosphere and hence to bipolar field-aligned currents
centered in the vortical flows closing in ionospheric Pedersen current. In the case of
southward IMF and high-latitude lobe reconnection the model predicts that the vortical
flows are displaced poleward of the open-closed field line boundary with reversed fieldaligned currents compared with the former case. During January 2004, a unique campaign
took place during which magnetic field and plasma instruments on board the CassiniHuygens spacecraft measured the in situ solar wind and embedded interplanetary magnetic
field while the Hubble Space Telescope simultaneously observed the far ultraviolet aurora
in Saturn’s southern hemisphere. The IMF was highly structured during this interval. The
electric potential at Cassini is estimated from solar wind magnetic field and velocity
measurements for the case of low-latitude or lobe reconnection. We show that a dayside
FUV signature of intense electron precipitation is found poleward of or along the main
oval during a period of minor compression period when the dayside reconnection voltage
is estimated to be 30–100 kV. Overall, we find that the conceptual model of Bunce et al.
(2005a) provides a good estimate of the UV brightness and power for the case of
northward IMF but somewhat underestimates the power for the southward IMF case,
except if the speed of the vortical flow is larger than its value in the nominal
model.
Citation: Gérard, J.-C., E. J. Bunce, D. Grodent, S. W. H. Cowley, J. T. Clarke, and S. V. Badman (2005), Signature of Saturn’s
auroral cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring, J. Geophys. Res., 110,
A11201, doi:10.1029/2005JA011094.
1. Introduction
[2] The Earth’s dayside cusp is the magnetospheric region where magnetosheath plasma has direct access to the
ionosphere. The magnetospheric cusp therefore plays an
important role as the region of most direct connection
between the ionosphere and the interplanetary medium
through reconnection. It exists whether the interplanetary
magnetic field is northward or southward. Reconnection
between magnetospheric and interplanetary magnetic field
1
Laboratoire de Physique Atmosphérique et Planétaire, Université de
Liège, Liège, Belgium.
2
Department of Physics and Astronomy, University of Leicester,
Leicester, UK.
3
Center for Space Physics, Boston University, Boston, Massachusetts,
USA.
Copyright 2005 by the American Geophysical Union.
0148-0227/05/2005JA011094$09.00
(IMF) lines is believed to occur whenever their directions
(or at least one component) are antiparallel [Onsager and
Fuselier, 1994; Fuselier et al., 1997]. During southward
IMF conditions, magnetic field lines in the subsolar region
can connect to the solar wind magnetic field and become
open. Reconnection can take place at the high-latitude
magnetopause during northward IMF conditions. During
intermediate conditions with a small northward and dominating east-west (By) component of the IMF, a mixed
situation with reconnection at the high- and the low-latitude
region may occur simultaneously [Reiff and Burch, 1985].
In situ measurements of precipitated particles have confirmed that the location and spectrum of particle precipitation in the cusp depend on the IMF orientation [Newell et
al., 1989] and that they are indicators of reconnection at the
magnetopause [Onsager et al., 1993].
[3] Milan et al. [2000] observed ultraviolet emission near
local noon, poleward of the dayside auroral oval, with the
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Figure 1. Polar view looking onto the northern ionosphere
with the pole at the center, in a frame which is fixed relative
to the Sun. The solid lines indicate plasma streamlines and
the circled dots and crosses indicate regions of field-aligned
current directed upward and downward, respectively. Taken
from Cowley et al. [2004].
UVI imager on the Polar satellite during a period of
northward IMF. They interpreted this feature as the signature of high-latitude reconnection and described its motion
in response to IMF By changes in coordination with observations of the large-scale convection flow. Frey et al. [2002]
showed that the Lyman-a proton auroral brightness at the
cusp footprint depends on the solar wind dynamic pressure.
They also concluded that the cusp magnetic local time
location depends on the IMF By component, consistent with
theoretical considerations of the partial penetration of the
IMF By into the magnetosphere [Cowley, 1981]. Fuselier et
al. [2002] showed that the spot-like cusp signature located
poleward of the oval merges with the dayside auroral oval
as the IMF turns southward. These results were discussed in
the context of antiparallel reconnection (at the high-latitude
lobe magnetopause during northward IMF) and possible
component reconnection (at the low-latitude magnetopause
during southward IMF). Several studies demonstrated that
the cusp footprint occurs at lower latitudes for southward
IMF Bz than for northward Bz and described associated
particle precipitation (latitude dispersion) and convection
(shift of convection boundaries) characteristics [McCrea et
al., 2000; Sandholt et al., 2001].
[4] Saturn’s aurora has only recently been investigated
with sufficient spatial and temporal resolution to address the
question of the presence of an optical signature of the
dayside cusp. Until far ultraviolet (FUV) observations with
the Hubble Space Telescope became available, most information concerning the morphology of the FUV aurora was
based on data collected with the Voyager UV spectrometer
(UVS) during the Voyager 1 and 2 Saturn flybys. The aurora
then appeared as a narrow ring at 80 latitude region with no
measurable emission inside the oval [Broadfoot et al., 1981;
Sandel and Broadfoot, 1981]. At Saturn, as at Jupiter, both
HI Lyman-a and H2 Lyman and Werner bands are present in
the polar regions of both hemispheres. Saturn’s aurora
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showed intensity temporal variations (factors of 2 – 5)
[Sandel et al., 1982], suggesting a solar wind controlled
aurora. Outbursts of Lyman-a were intermittently observed
with the International Ultraviolet Explorer (IUE) over a
decade with a brightness of a few kiloRayleighs (kR)
[Clarke et al., 1981; McGrath and Clarke, 1992], also not
inconsistent with solar wind control of Saturn’s aurora. The
Faint Object camera on board the Hubble Space Telescope
(HST) was used to obtain the first image of the north aurora
[Gérard et al., 1995]. FUV images [Trauger et al., 1998]
collected with the Wide Field Planetary Camera (WFPC2)
showed a northern auroral arc appearing mostly fixed in
local time near the dawn limb but with variable brightness.
Images of the south aurora were obtained with the Space
Telescope Imaging Spectrograph (STIS) between 11 October
1997 and 29 January 2004 [Gérard et al., 2004; Cowley et al.,
2004; Clarke et al., 2005; Grodent et al., 2005]. They showed
the presence of an auroral oval extending continuously from
the midnight sector via dawn into the postnoon hours, though
strongly varying in brightness with local time. The dayside
main oval lies between 70 and 80 and is generally brighter
and thinner in the morning than in the afternoon sector. The
afternoon sector is characterized by more diffuse emission
at higher latitudes. Weak emission is also observed poleward of the main oval up to the pole. A spiral structure of
the main oval was occasionally seen. The brightness of the
main oval ranges from below the STIS threshold of 1 kR
of H2 emission up to 75 kR. The total electron precipitated power varies between 20 and 140 GW, which is
comparable to the Earth’s active aurora but about two
orders of magnitude less than on Jupiter. Comparison of
spectra with a synthetic model of electron excited H2 were
used to estimate the energy of the primary auroral electrons as 12 ± 3 keV [Gérard et al., 2004], based on a lowlatitude model atmosphere relying on Voyager occultation
measurements.
[5] Cowley et al. [2004] have suggested that Saturn’s
aurora is associated with a ring of upward current flowing
along the open-closed field line boundary generated by the
difference in angular velocity between open and outer
magnetosphere closed field lines. In Figure 1 we show a
view of the flows and currents which are present in Saturn’s
ionosphere, associated with the three main flow regimes
discussed previously by Hill [1979], Vasyliunas [1983], and
Dungey [1961]. The three flow regions are as follows: (1) a
lower-latitude region which maps to the subcorotating
plasma in the equatorial plane, associated with a fieldaligned current system which is of insufficient magnitude
and flowing at the wrong colatitude to account for the main
oval auroras at Saturn [Cowley and Bunce, 2003], (2) a
higher-latitude region of subcorotating flows where closed
field lines are stretched out down-tail and eventually pinch
off, forming a plasmoid, which is subsequently released
downtail (the ‘‘Vasyliunas cycle’’), and (3) a region of flow
which is driven by reconnection at the dayside magnetopause in which ‘‘open’’ field lines mapping to the tail lobes
flow antisunward over the poles, and following reconnection in the tail, return to the dayside, drawn here principally
via dawn, in a single cell convection pattern (the ‘‘Dungey
cycle’’). It is the upward directed field-aligned current along
the boundary between open and closed field lines (shown by
the inner dashed circle), which is suggested to be associated
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with the main auroral oval at Saturn, possibly augmented by
more diffuse emissions in the region immediately equatorward, due to hot plasma production and precipitation in the
Dungey and Vasyliunas cycles [Cowley et al., 2005]. In this
case, the main oval aurora is expected to be strongly
modulated by the upstream solar wind conditions, as will
be discussed in section 2 with respect to the coordinated
Cassini-HST observations.
[6] Gérard et al. [2004] have suggested that dayside
UV emissions in Saturn’s polar ionosphere show observational evidence of the kronian ‘‘cusp’’ in the noon
sector. They described the emission as presenting two
distinct states. The first is a bright arc-like feature located
in the prenoon sector that appears as an intensification of
the main oval. The second is a more diffuse ‘‘spot’’ of
aurora lying poleward of the general location of the main
auroral oval, possibly related to different upstream interplanetary magnetic field (IMF) orientations. Bunce et al.
[2005a] took up the suggestion that these emissions
correspond to the cusp. They showed that direct precipitation of electrons in the cusp regions is not capable of
producing significant UV aurora. Instead, they investigated
the possibility that the UV emissions described by Gérard
et al. [2004] are associated with reconnection occurring at
the dayside magnetopause, possibly pulsed, analogous to
flux transfer events seen at the Earth. As discussed for
the case of the Jovian cusp by Bunce et al. [2004],
pulsed reconnection at the low-latitude dayside magnetopause for the case of northward IMF will give rise to
pulsed twin-vortical flows in the magnetosphere and
ionosphere in the vicinity of the open-closed field-line
boundary and hence to bipolar field-aligned currents
centered in the vortical flows which close at one end in
ionospheric Pedersen currents and at the other end in the
Chapman-Ferraro currents on the magnetopause. In the
case of southward IMF and high-latitude lobe reconnection, Bunce et al. [2005a] also expect that the vortical
flows are displaced poleward of the open-closed field line
boundary such that the field-aligned currents are also
reversed compared with the former case. These perturbation currents associated with pulsed reconnection at the
magnetopause will straddle (for the case of northward
IMF) or sit slightly poleward of (for the case of southward IMF) the open-closed field line boundary associated
with the main auroral oval. In addition to the variability
in flows and currents expected to be present due to northsouth fluctuations in the IMF, modulation is also introduced by the large-scale structure of the solar wind and
IMF at Saturn’s orbit due to corotating interaction regions
(CIRs) [Jackman et al., 2004]. Consequently, Bunce et al.
[2005a] also considered a slow and a fast flow model to
account for the highly developed nature of CIRs at
Saturn’s distance from the Sun, causing the solar wind
and IMF to be separated into distinct regions of rarefaction and compression.
[7] During January 2004, a unique campaign took place
during which magnetic field, plasma, and radio wave
instruments on board the Cassini-Huygens spacecraft measured the in situ solar wind and embedded IMF while HST
simultaneously observed the far ultraviolet aurora in Saturn’s southern hemisphere. Results concerning the
correlations between the main oval and the solar wind
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characteristics, the shape of the auroral region, its level of
corotation, and brightness distribution were described by
Clarke et al. [2005], Crary et al. [2005], Cowley et al.
[2005], Bunce et al. [2005b], and Grodent et al. [2005]. The
correlation between the auroral FUV emission and kilometric radio emission was analyzed by Kurth et al. [2005].
In this study we describe the main features of the FUV
signature of cusp precipitation in these data and relate it to
the characteristics of the IMF measured upstream with the
Cassini orbiter en route to Saturn. We compare measured
brightness with the energy fluxes expected from the model
of Bunce et al. [2005a].
2. Observations
2.1. HST Images
[8] A total of 68 STIS images of Saturn’s auroras,
covering 17 HST orbits, were obtained with the photoncounting detector 25MAMA (Multi-Anode Micro channel
Array) in the period between 8 and 30 January 2004
during the Cassini-HST campaign. The 24.7 24.7 arcsec2
field of view of STIS includes the full planet’s disk and a
fraction of the ring system. Two imaging modes were used
for this data set. In the clear mode, the passband extends
from 115 to 180 nm and is sensitive to the H2 Lyman and
Werner bands as well as the strong H Lyman-a line. In the
‘‘filtered’’ mode, an SrF2 filter was added in the optical
path, rejecting Lyman-a. The MAMA array has 1024 1024 pixels with a linear size of 0.0243 arcsec, thus
providing a field of view of 24.7 24.7 arcsec2 with a
0.08 arcsec full width at half maximum point spread
function. The effective spatial resolution projected at
Saturn is thus approximately 500 km. For a feature located
at the central meridian longitude (CML) the smearing
introduced by the planetary rotation is 0.56 in longitude
in 1 min. The procedure used to determine the position of
the center of the planet and the coordinates of each pixel
was described by Gérard et al. [2004] and Grodent et al.
[2005]. It is thus possible to generate polar (orthographic)
projections. As part of the data reduction process, the
images were flat-fielded using the most recent low-order
flat field and pixel-to-pixel files provided by the Space
Telescope Science Institute (STScI). Dark counts were
subtracted using the appropriate STScI dark files. Maps
were built in a system fixed relative to the central
meridian, simulating an observer looking from above the
north pole through the planet.
[9] Each orbit consists of four consecutive images. The
first visit (V1) or ‘‘full rotation visit’’ comprised five
consecutive HST orbits. The other visits (V2 to V13) all
consist of one orbit approximately every 2 days. The clear
images were accumulated for 270 s and the filtered images
were obtained with exposure times varying from 640 s to
740 s. A first filtered image was followed by two ‘‘clear’’
exposures and another filtered image. The full list of
observations was given by Grodent et al. [2005]. During
this campaign, the sub-Earth planetocentric latitude changed
from 25.65 to 25.98, and the subsolar longitude remained
within 1 of the sub-Earth longitude. Table 1 summarizes
the times and CMLS of the 13 visits. The variations of the
global morphology, brightness, and their relations with in
situ solar wind measurements by Cassini were described by
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Clarke et al. [2005], Cowley et al. [2005], Bunce et al.
[2005b], more detailed by Grodent et al. [2005], and are
summarized hereafter.
2.2. In Situ Solar Wind Measurements
[10] During this campaign, the structure of the interplanetary medium was measured by the Cassini spacecraft en
route to Saturn. During this period, Cassini was located
0.2 AU upstream of Saturn in the radial direction and was
off the planet-Sun line by 0.5 AU toward dawn. Using an
average solar wind speed of 500 km s1, the radial
propagation delay from Cassini to Saturn is 17 hours
[Crary et al., 2005]. Jackman et al. [2004] find that the data
during the Cassini approach phase to Saturn show that the
interplanetary magnetic field (IMF) structure generally
consists of two sectors during each rotation of the Sun,
with crossings of the heliospheric current sheet embedded
within few-day high field compression regions, surrounded
by several day rarefaction regions. This pattern is consistent
with that expected to be generated by recurrent CIRs during
the declining phase of the solar cycle. The top four panels of
Figure 2 show the components of the magnetic field in the
RTN (radial, tangential, normal) coordinate reference system and the total magnitude measured by the Cassini
magnetometer during the interval 1 – 31 January 2004.
RTN is a right-handed spherical polar system referenced
to the Sun’s spin axis, in which BR is positive radially
outward from the Sun toward the spacecraft, BT is the
azimuthal component positive in the direction of planetary
motion, and BN is the component positive northward in the
solar equatorial plane. Note that the orientation of Saturn’s
magnetic poles is reversed compared to the Earth’s case.
The fifth panel shows an estimate of the low-latitude
dayside reconnection voltage (in units of kV) across Saturn’s magnetopause associated with the production of open
flux based by analogy with the results of studies made at the
Earth [Jackman et al., 2004]. In the estimate of Jackman et
al. [2004], large low-latitude reconnection voltages are
favored for northward pointing IMF at Saturn, reversed to
the case for Earth due to the opposite sense of the planetary
field. The expression for the voltage associated with open
flux production is
FLL
q
;
¼ Vsw B? L0 cos
2
4
ð1Þ
where Vsw is the radial speed of the solar wind, B? is the
strength of the IMF component perpendicular to the radial
flow, L0 is a scale length assumed to be 10 RS (where 1 RS =
60228 km) by analogy with the Earth, and theta is the clockangle of the IMF relative to Saturn’s northern magnetic axis.
The sixth panel in Figure 2 indicates an estimated voltage
calculated for high-latitude (lobe) reconnection. In this case,
large high-latitude reconnection voltages are associated with
southward pointing IMF. This formula is given by
1
q
FHL ¼ Vsw B? L0 sin4
;
2
2
ð2Þ
where the cos variation in the original northward IMF
variation has been replaced with a sin dependence, thus
producing large voltages for intervals of southward pointing
IMF Bz. These two different scenarios will be referred to
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henceforth in this paper as the voltages associated with lowand high-latitude reconnection, respectively. The overall
magnitude of the voltage associated with high-latitude
reconnection is also estimated to be roughly half of that
associated with low-latitude reconnection (see discussion by
Bunce et al. [2005a]). These two parameters act together to
indicate the level of dayside reconnection that may reasonably be expected to be present if the field which is measured
at Cassini is the same as the field which impinges upon
Saturn. However, as discussed above, the spacecraft is
located at 0.5 AU off the planet-Sun line (corresponding
to 3 of heliographic longitude). Therefore the field
magnitude measured at Cassini is used here as an indication
of the magnitude of the field present at Saturn, but the short
timescale fluctuations in the BN component (i.e., IMF Bz)
cannot be reliably propagated. We have thus overplotted a
simplified ‘‘maximum’’ voltage (seen by the upper most
black lines in each of panels 5 and 6), which simply
estimates the reconnection voltage from the magnitude of
the field assuming a purely northward or southward IMF
direction in each case. It is these potentials which will now
be discussed in the context of the HST observations. Using a
17-hour propagation delay, the HST visits are indicated in
Figure 2 by the vertical dashed lines similarly marked V1 to
13. As discussed by Crary et al. [2005], the timing in this
delay relative to the interplanetary data is unknown to
within several hours, due to nonradial propagation effects
and the separation of the spacecraft and the planet in
heliographic longitude.
[11] The highly structured nature of the IMF was discussed in detail for this interval by Bunce et al. [2005b].
The HST observations took place in a period when different
solar wind regimes may be distinguished. Visits V1 –5 were
made during a low-field rarefaction interval, which followed a major compression during 1 – 5 January. Here the
total IMF strength is typically <0.1 nT, the maximum lowlatitude dayside reconnection voltage (for a purely northward IMF as described above) is 20 kV (10 kV on
average for the maximum high-latitude reconnection voltage), and the solar wind dynamic pressure (when available)
is 0.003 nPa [Crary et al., 2005]. HST images show a
correspondingly ‘‘quiet’’ auroral oval, which remains
throughout the low solar wind activity interval. Cowley et
al. [2005] indicate that the decreasing size of the auroral
oval over this time period is suggestive of a nightside
reconnection rate stronger than the dayside rate, thus closing
flux at a greater rate than it can be opened.
[12] Visits V6– 10 took place after a ‘‘minor’’ compression event on 17 January, when the field strength was
typically 0.2– 0.4 nT, the maximum low-latitude dayside
reconnection voltage was 50– 100 kV, and the solar wind
dynamic pressure was 0.01 nPa. The solar wind density
was 0.03 cm3, the solar wind velocity was decreasing,
and the reconnection voltages were correspondingly ‘‘medium’’ in strength. The maximum high-latitude reconnection voltage estimate gives on average 40 kV, with peaks of
up to 70 kV. During this period, the oval is first contracted
between V5 and 6 and is significantly brighter than the
previous five images and appears to form a spiral. Cowley et
al. [2005] suggested that this contraction of the auroral oval
and enhanced precipitation are indicative of an episode of
strong tail reconnection and the closure of open flux in the
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Figure 2. Plot of the Cassini magnetic field measurements. The first four panels show the components
of the magnetic field in RTN coordinates BR, BT, BN, and the magnitude of the field jBj in units of nT. The
fifth and sixth panels show the estimated reconnection voltage in units of kV, derived from the magnetic
field and the velocity data as described in the text. The high-frequency trace shows the electric potential
(kV) calculated for low-latitude dayside reconnection and high-latitude reconnection in panels 5 and 6
respectively. The uppermost smooth lines in each panel indicate a simple ‘‘maximum’’ voltage in each
case, as described in the text. The dashed vertical lines indicate the approximate times of the HST images,
shifted by 17 hours. Adapted from Bunce et al. [2005b].
tail. During the ‘‘medium’’ activity interval V6 –10, the oval
slowly expands, indicating open flux is added on the
dayside at a faster rate than was being closed on the
nightside.
[13] Finally, visits V11 –13 took place just following the
start of a major compression period observed during 25–
30 January, when the field strength increased to 0.5–
2.0 nT, the estimated maximum low-latitude dayside
reconnection voltage was typically 400 kV (200 kV
for the estimate of the maximum high-latitude reconnection
voltage), and the solar wind dynamic pressure was on
average increased to 0.03 nPa. In this final period of
HST observations, very dynamic and bright auroral emission was observed. This interval coincides with the major
compression region that began on 25 January with the
forward shock wave marking the start of the CIR compres-
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sion region. Images obtained on V11 show that the dark
polar cap has been filled in with bright aurora. According to
Cowley et al. [2005], this is an early effect of the onset of
tail reconnections triggered by the sudden compression of
the magnetosphere. Following this, V13 showed a bright,
expanding oval.
[14] In summary, one can distinguish four time intervals
over the period of coordinated Cassini-HST observations: a
low-field rarefaction region from 8 to 16 January following
a major compression event, a ‘‘minor’’ compression event
on 17 January, a rarefaction region with intermediate field
strengths from 19 to 25 January, and a major compression
region from 26 to 30 January. The general morphology of
the main oval can be understood in terms of the characteristics of the solar wind at Saturn and the balance between
the rate of dayside IMF reconnection and nightside field line
reconnection.
3. Cusp-Solar Wind Relationship
[15] Enhanced precipitation in the noon sector is not
necessarily a signature of the dayside cusp. The sequence
of observations performed during V1 shows that ‘‘blobs’’ of
plasma carried by convection may easily be confused with
the cusp enhancement if the time evolution of the global
morphology is not available. We thus set criteria to discriminate between the two situations. A first condition is
that the cusp emission must remain nearly fixed in local
time over the 45 min of observing time of an HST orbit.
Instead, plasma ‘‘blobs’’ are observed to move eastward at
70% of the corotation angular velocity. Since four STIS
images were collected during each HST orbit, the azimuthal
eastward motion is expected to be 15 over the 35 min
separating the first and the last images of each orbit.
Therefore we only consider cases when the noon sector
enhancement moves eastward by less than 5. We also set a
brightness threshold on the identification of cusp features.
They must clearly exceed the background level by at least a
factor of two.
[16] Figure 3 shows examples of filtered STIS images
collected at three different periods of the January 2004
campaign. The first image (Figure 3, V2) was obtained on
10 January (V2). At this time, the IMF magnitude was
0.05 nT and BN was very small. Both the maximum lowand high-latitude reconnection potentials were below 20 kV.
Figure 3. Polar (orthographic) projections of HST-STIS
images of Saturn’s south FUV aurora. Images were taken on
(top) 10 January (V2), (middle) 21 January (V8), and
(bottom) 28 January (V12) 2004 and belong to the low-field
rarefaction period, the recovery period following the
15 January solar wind compression, and the recovery period
following the 25 January interplanetary shock, respectively.
The image has been projected on a polar view where the south
is seen by a virtual observer above the north pole, looking
through the planet. In these viewing conditions, the south pole
is at the center, the direction of the Sun (1200 LT) is
conveniently oriented towards the bottom, dawn (0600 LT) to
the left, dusk (1800 LT) to the right as indicated in the LTclock
panel. The parallels are separated by 10 of latitude and the
meridians by 10 of longitude. See color version of this figure
at back of this issue.
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Figure 4. Polar projections of HST-STIS images of Saturn’s southern hemisphere UV aurora taken on
21 January 2004 (Visit 8). (a) and (d) Images were taken in the SrF2 ‘filtered’ mode; (b) and (c) images
were unfiltered. See color version of this figure at back of this issue.
Solar wind conditions were thus inappropriate to produce
significant dayside reconnection and cusp precipitation. The
auroral oval is wide, which is characteristic of steady state
solar wind conditions encountered during rarefaction periods
(large open field line flux in the polar cap, no dayside
reconnection). As expected, no enhancement is observed
along the oval or poleward in this case. Figure 3 (V8) shows
an image collected on 21 January (V8). Unlike the previous
case, it shows a significant enhancement located near 1200 LT
poleward of the main auroral oval. This cusp spot is comparatively as bright as the dawn sector oval where limb brightening probably increases the vertical intensity relative to
features in the noon sector. This time period corresponds to
the interval following the minor compression in the solar
wind when the IMF intensity was about 0.5 nT with the field
mostly directed in the sense of positive Bt (negative By) with
fluctuations of the north-south component on short timescales. The maximum reconnection voltages at this approximate time for low-latitude and high-latitude reconnection are
80 kV and 40 kV, respectively, the former being associated with the largest intensities for any of the HST images
collected during the campaign, with the exception of V11, 12,
and 13 when the enhancement of the main oval and the
perturbed morphology masks out any signature of cusp
emission The location of this cusp emission poleward of the
main auroral oval is thus consistent with an interval of
southward IMF and high-latitude reconnection. The full set
of four STIS images collected during V8 is shown in Figure 4.
The first filtered image is identical to Figure 3b. The scales
of the first and fourth images have been adjusted to account
for the higher sensitivity of the clear mode compared to the
other two images with the SrF2 filter. The images, together
with the coordinates of the spot in the noon sector given in
Table 1, indicate that the spot center is consistently poleward
of the main oval location and quasi-stationary in local time.
The oval at this time exhibits a spiral structure indicative of
active tail reconnection at a rate exceeding that of dayside
reconnection. Finally, the image in Figure 3 (V12) was taken
on 28 January (Visit 12), following the start of the major
compression. The oval has a significantly lower size than
previously, but it is also significantly thicker, especially in the
morning and noon sectors. No cusp emission is observed
poleward of the main oval in this case.
[17] In summary, a clear signature of cusp precipitation
poleward of the oval is observed following the minor compression period when the maximum high-latitude reconnection voltage is a few tens of kV and is not obscured by other
dynamical dominant processes. Images collected during V7
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Table 1. Time and Position of Near-Noon Auroral Brightening
Visit
File
Date (January 2004)
Subsolar Long.,a deg
Time, UT
CML
Lat., deg
Long., deg
Average UV, kR
V1
V2
V3
V4
V5
V6
V7
V8
V9
V10
V11
V12
V13
o8wi02c69
o8wi01s19
o8wi03dsq
o8wi04rtq
o8wi05h2q
o8wi06baq
o8wi07k0q
o8wi08b5q
o8wi09moq
o8wi10bdq
o8wi11huq
o8wi12qwq
o8wi13llq
8
10
12
14
16
18
20
21
23
24
26
28
30
38.8 – 271.3
219.5 – 236.9
202.8 – 220.2
221.8 – 239.8
295.0 – 312.9
224.2 – 242.2
297.7 – 315.7
316.9 – 334.9
317.4 – 335.4
336.5 – 354.5
355.5 – 13.5
302.9 – 320.9
357.7 – 15.6
0442
0441
0929
0441
0129
0440
0128
2040
0439
2351
1903
0127
1901
49.2
230.1
213.7
233.5
306.9
236.4
310.1
329.5
330.2
349.5
8.7
316.3
11.3
—
—
76.1
75.9
77.9
—
79.2b
79.8b
76.6
76.5
—
—
—
—
—
170
177
190
—
177
175
180
160
—
—
—
—
—
50
23
9
—
14
21
20
29
—
—
—
UV Power, GW
6.5
4.3
2.0
—
2.1
5.0
2.5
2.5
a
Subsolar latitudes range from 25.45 to 25.57.
Indicates the presence of a cusp feature detached from the main oval.
b
(19 – 20 January) also exhibit a cusp-like signature (Figure 5)
fixed in local time. However, the filtered images show that in
this case, the noon feature is more elongated (along a meridian
5 poleward of the oval) than in V8. An intensity gap is
clearly observed at 1100 LT. The cusp arc remains poleward
of the morning sector location of the main oval and slowly
fades out until it disappears at dusk. As mentioned before,
cusp signatures at the latitude of or close to the main oval are
more difficult to visually identify, as they may be local
enhancements of the oval connected to subcororating
plasma blobs. An example is shown in Figure 6 illustrating observations performed during V9. In this series, the
intensification of the oval observed near 1200 LT is
quasi-stationary. At this time, the IMF intensity was about
0.4 nT and was directed nearly in the ecliptic plane. We
now compare the cusp emission identified in Figures 4
Figure 5. Polar projections of HST-STIS images of Saturn’s southern hemisphere UV aurora taken on
20 January 2004 (Visit 7). See color version of this figure at back of this issue.
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Figure 6. Polar projections of HST-STIS images of Saturn’s southern hemisphere UV aurora taken on
23 January 2004 (Visit 9). See color version of this figure at back of this issue.
and 6 (V8 and 9) with the theoretical model of low- and
high-latitude dayside reconnection presented by Bunce et
al. [2005a].
4. Comparison of FUV Brightness With the Cusp
Model
[18] As described in the introduction, Bunce et al. [2005a]
have recently developed a conceptual model of the vortical
flows and currents which arise due to reconnection, possibly
pulsed, on the dayside magnetopause at Saturn. Here we
now compare the HST-STIS observations of the candidate
dayside cusp aurora described in the previous section, with
the upstream in situ interplanetary medium sampled by the
Cassini spacecraft en route to Saturn. In order to augment
our understanding of Saturn’s cusp processes, we now
compare the UV observations made by the HST with the
cusp model of Bunce et al. [2005a], which is appropriate to
the solar wind conditions measured during the January
interval.
[19] We will compare the two examples of what we
understand to be dayside cusp related emission described
in the previous section and shown in Figures 4 and 6,
namely those identified on 22 January (V8) and
23 January (V9), with the theoretical model of Bunce et
al. [2005a]. V8 is thought to be an example of a highlatitude ‘‘spot’’ indicative of southward IMF Bz. As
discussed in the previous section, these data were collected during the interval of ‘‘intermediate’’ field strength,
following the minor compression region that passed the
spacecraft on 16 January. This level of intermediate field
strength (which is on average 0.5 nT) was modeled by
Bunce et al. [2005a] as being their ‘‘slow flow’’ model.
As recently discussed by Bunce et al. [2004, 2005a],
pulsed reconnection at the low-latitude dayside magnetopause for the case of northward IMF will give rise to
pulsed twin-vortical flows in the magnetosphere and
ionosphere in the vicinity of the open-closed field-line
boundary (shown by the inner dashed circle in Figure 1)
and hence to bipolar field-aligned currents centered in the
vortical flows which close at one end in ionospheric
Pedersen currents and at the other in the ChapmanFerraro currents on the magnetopause. In the case of
high-latitude lobe reconnection for southward IMF, we
also expect to have pulsed twin-vortical flows and
corresponding bipolar field-aligned currents. However,
the vortical flows in this case are displaced poleward of
the open-closed field line boundary and are reversed in
sense, such that the field-aligned currents are also reversed compared with the former case. The upward
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Table 2. Predicted Cusp Parameters Using the Conceptual Model of Bunce et al. [2005a]
Visit 8
Areaa 8 1012 m2
Visit 9
Area 6 1012 m2
Flow Factor Fb
Voltage,c kV
Ave FAC,d mA m2
Total Current,e MA
Average UV,f kR
UV Power,g GW
1
2
4
6
1
2
4
6
59
118
238
358
110
222
446
668
0.007
0.013
0.027
0.040
0.127
0.151
0.199
0.248
0.05
0.11
0.21
0.32
0.80
0.95
1.26
1.56
0.7 (0.07)
2.8 (0.3)
11.3 (1.1)
25.4 (2.5)
18.4 (2.7)
28.9 (4.2)
60.4 (8.7)
105.5 (15.2)
0.06
0.24
1,0
2.2
1.3
2.1
4.3
7.6
a
The total area, equatorward and poleward of the merging gap, considered in the model calculation.
The factor by which the vortical flow speeds, V2 are increased.
c
The implied voltage required to increase the vortical flow speeds by the factor given in footnote b.
d
The magnitude of the field-aligned current density averaged over the area, obtained from the total current divided by the area defined in footnote a.
e
The total upward directed field-aligned current integrated over the area defined in footnote a. Any small region of downward current in the area is not
included.
f
The averaged emission rate in the FUV (B and C singlet H2 states), obtained from the averaged precipitating energy flux. The values in brackets indicate
the subsolar or high-latitude sheath plasma values in the IMF Bz > 0 and IMF Bz < 0 models, respectively.
g
The total power emitted in the FUV from the area given in footnote a, obtained as 20% of the total precipitating particle power.
b
currents which lie along the open-closed field line boundary are thought to be responsible for the production of
the main auroral oval at Saturn [Cowley et al., 2004]. For
the case of northward IMF Bz the pulsed currents will
modulate the general upward field-aligned current around
the boundary, while for the case of southward IMF the
pulsed currents are poleward of the main auroral oval
currents. Bunce et al. [2005a] have also modeled the
azimuthal asymmetries associated with IMF By, but this
will not be addressed in this paper.
[20] In Table 2 we show the results from the (modified)
model of Bunce et al. [2005a]. In each case (V8 and 9)
we start with the ‘‘slow model’’ which, as discussed by
Bunce et al. [2005a] is intended to represent intermediate
IMF strengths observed at this time. In the model the
vortical flows associated with reconnection are taken to
be elliptical in form, and centered at either end of the
merging gap. The parameter V2 (in their equation (8a))
represents the strength of the vortical flows and has then
been chosen such that the total voltage along the merging
gap associated with the flow is 100 kV in the case of
northward IMF Bz and 50 kV in the case of southward
IMF Bz. We have simply modified the strength of the
vortices here by introducing a flow factor F = 1, 2, 4, 6,
to match the UV brightness and power with that measured during V8 and 9. The second column indicates the
voltage (kV) associated with the vortices augmented by
the factors given in the first column. The voltages shown
in Table 2 can be compared with those we have estimated
based on the Cassini interplanetary data shown in
Figure 2. The third column shows the averaged fieldaligned current density over the emission area (see
below), and the fifth and sixth columns give the averaged
UV brightness (kR) and the total power output (GW).
The areas in the model are chosen by looking at the
field-aligned current density contours in the model that
are above a simple background level (0.05 mA m2),
over which we integrate to derive the parameters in
Table 2. For the high-latitude case (V8), in the southward
IMF Bz model, we obtain an area of 4 1012 m2 for both the
plasma mantle region (the high-latitude lobe population
equatorward of the merging gap containing plasma originating from the magnetosheath), and the same area in the high-
latitude magnetosheath region (the population poleward of
the merging gap) giving a total area of 8 1012 m2. For the
case of the subsolar reconnection (V9) we consider a total area
of 6 1012 m2. This is divided evenly between the region
equatorward of the open-closed field line boundary, in the
magnetospheric plasma regime, and the region poleward of
the open-closed field line boundary in the magnetosheath
plasma regime.
[21] As indicated in Table 2, for V8, a factor of 6 in
the slow model is required to produce an average UV
brightness of 26 kR and a total UV power of
2.25 GW, to be compared with the HST observed
values of 21 kR and 5 GW. However, this requires a
voltage value of >200 kV, which is approximately a
factor of 4 higher than the maximum high-latitude reconnection voltage estimated at this time (and is also the
same as the standard slow model). For V9, the model
agrees well and we obtain an average UV brightness of
18 kR and a total UV power of 1.3 GW using the
standard slow model (where flow factor F = 1). This
compares to the HST observed values of 20 kR and
2.5 GW. The standard slow model of Bunce et al. [2005a] for
the case of northward IMF requires peaks of 100 kV, in fair
agreement with the maximum low-latitude reconnection
voltage during V9. Overall, we find that the conceptual model
of Bunce et al. [2005a] provides a reasonable estimate of the
UV brightness and power for the two examples of UV cusp
emission presented here. However, the model agrees significantly better for the case indicative of low-latitude
reconnection than for the case thought to be associated with
high-latitude reconnection.
[22] The question whether the reconnection is steady or
pulsed is not a significant factor in this study, as the
model represents a snapshot of the flow and current at a
particular time. It can in principle represent a steady
situation or one that is evolving. For low-latitude reconnection the pattern is probably part of an evolving pattern
at an early phase of a burst when the flow is reasonably
localized around the merging gap and before it is spread
over the whole polar region. The assumption of a pulsed
flow in this context also allows to increase the instantaneous voltage to some factor larger than the expected
mean value. However, for high-latitude reconnection, one
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might reasonably expect a more compact flow system
poleward of the open-closed field line boundary anyway,
so whether the instantaneous flow pattern analyzed is part
of an evolving pulsed system or a more steady situation
is difficult to determine.
5. Conclusion
[23] Observations of Saturn’s FUV south aurora were
made with the Hubble Space Telescope in parallel with in
situ measurements of the solar wind plasma characteristics
and embedded interplanetary magnetic field as the Cassini
probe was approaching Saturn. The IMF was highly
structured during this interval. The dayside reconnection
voltage across Saturn’s magnetopause associated with the
production of open flux and lobe reconnection is calculated
from the solar wind IMF and speed by analogy with the
scenario at Earth. A dayside FUV signature of intense
electron precipitation similar to that previously identified
by Gérard et al. [2004] is found poleward of or along the
main oval following the minor compression period when
the IMF intensity was a few tenths of nT and the potential
rapidly varied in the range 30– 100 kV. Uncertainties on
the time delay between the HST images and the arrival of
the solar wind plasma at Saturn’s magnetopause prevent
accurate relative time positioning of aurora and the solar
wind changes. However, these results are in good general
agreement with the conceptual model by Bunce et al.
[2005a] where pulsed reconnection at the low-latitude
dayside magnetopause is expected to give rise to pulsed
twin-vortical flows in the magnetosphere and hence to
bipolar field-aligned currents centered in the vortical flows
when the IMF is directed northward. In the case of
southward IMF and high-latitude lobe reconnection, the
model predicts that the vortical flows are displaced poleward of the open-closed field line boundary with reversed
field-aligned currents compared with the former case. The
slow model gives good agreement in the case of lowlatitude reconnection, assumed to be present during intervals of northward IMF Bz but does not well reproduce the
FUV brightness and power for high-latitude lobe reconnection, assumed to be present during intervals of southward IMF Bz. This indicates that lobe reconnection is
perhaps more efficient than Bunce et al. [2005a] have
assumed or that other mechanisms may be present. Further
observations and modeling work will be required to
improve our understanding of this complex system.
[24] Acknowledgments. This work is based on observations with the
NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute (STScI), which is operated by the AURA, Inc. for NASA.
JCG and DG acknowledge support from the Belgian Fund for Scientific
Research (FNRS). The PRODEX program of ESA provided financial
support for this research to the University of Liège. This research was also
funded by grants from the Space Telescope Science Institute and from
NASA to the University of Michigan. SWHC was supported by PPARC
Senior Fellowship PPA/N/S/2000/00197.
[25] Arthur Richmond thanks Harald Frey and George Gladstone for
their assistance in evaluating this paper.
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Figure 3. Polar (orthographic) projections of HST-STIS
images of Saturn’s south FUV aurora. Images were taken on
(top) 10 January (V2), (middle) 21 January (V8), and
(bottom) 28 January (V12) 2004 and belong to the low-field
rarefaction period, the recovery period following the
15 January solar wind compression, and the recovery
period following the 25 January interplanetary shock,
respectively. The image has been projected on a polar view
where the south is seen by a virtual observer above the north
pole, looking through the planet. In these viewing
conditions, the south pole is at the center, the direction of
the Sun (1200 LT) is conveniently oriented towards the
bottom, dawn (0600 LT) to the left, dusk (1800 LT) to the
right as indicated in the LT clock panel. The parallels are
separated by 10 of latitude and the meridians by 10 of
longitude.
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Figure 4. Polar projections of HST-STIS images of Saturn’s southern hemisphere UV aurora taken on
21 January 2004 (Visit 8). (a) and (d) Images were taken in the SrF2 ‘filtered’ mode; (b) and (c) images
were unfiltered.
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Figure 5. Polar projections of HST-STIS images of Saturn’s southern hemisphere UV aurora taken on
20 January 2004 (Visit 7).
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Figure 6. Polar projections of HST-STIS images of Saturn’s southern hemisphere UV aurora taken on
23 January 2004 (Visit 9).
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